Multi-beam inspection apparatus with improved detection performance of signal electrons

ABSTRACT

The present disclosure proposes a crossover-forming deflector array of an electro-optical system for directing a plurality of electron beams onto an electron detection device. The crossover-forming deflector array includes a plurality of crossover-forming deflectors positioned at or at least near an image plane of a set of one or more electro-optical lenses of the electro-optical system, wherein each crossover-forming deflector is aligned with a corresponding electron beam of the plurality of electron beams.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of U.S. application 62/641,204 whichwas filed on Mar. 9, 2018, and which is incorporated herein by referencein its entirety.

TECHNICAL FIELD

The embodiments provided herein disclose a charged particle apparatuswith multiple charged particle beams, and more particularly, anapparatus utilizing multiple charged particle beams to observe orinspect scanned regions of an observed area of a sample surface.

BACKGROUND

When manufacturing semiconductor integrated circuit (IC) chips, patterndefects and/or uninvited particles (residuals) inevitably appear on awafer and/or a mask during fabrication processes, thereby reducing theyield to a great degree. For example, uninvited particles may betroublesome for patterns with smaller critical feature dimensions, whichhave been adopted to meet the more and more advanced performancerequirements of IC chips.

Currently, pattern inspection tools with a single electron beam are usedto detect the defects and/or uninvited particles. These tools typicallyemploy a scanning electron microscope (SEM). In the SEM, a beam ofprimary electrons having a relatively high energy is decelerated to landon a sample at a relatively low landing energy and is focused to form aprobe spot thereon. Due to this focused probe spot of primary electrons,secondary electrons will be generated from the surface. By scanning theprobe spot over the sample surface and collecting the secondaryelectrons, pattern inspection tools may obtain an image of the samplesurface.

SUMMARY

The embodiments of the present disclosure provide a charged particleapparatus with multiple charged particle beams, and more particularly,an apparatus utilizing multiple charged particle beams to observe orinspect scanned regions of an observed area of a sample surface.

In some embodiments, a crossover-forming deflector array of anelectro-optical system for directing a plurality of electron beams ontoan electron detection device is provided. The crossover-formingdeflector array includes a plurality of crossover-forming deflectorspositioned at or at least near an image plane of a set of one or moreelectro-optical lenses of the electro-optical system, wherein eachcrossover-forming deflector is aligned with a corresponding electronbeam of the plurality of electron beams.

In some embodiments, an electro-optical system for projecting aplurality of secondary electron beams from a sample onto respectiveelectron detection surfaces in a multi-beam apparatus is provided. Theelectro-optical system includes a plurality of crossover-formingdeflectors configured to create a crossover area on a crossing plane forthe plurality of secondary electron beams, wherein eachcrossover-forming deflector of the plurality of crossover-formingdeflectors is associated with a corresponding secondary electron beam ofthe plurality of secondary electron beams. The electro-optical systemcan also include a beam-limit aperture plate with one or more apertures,positioned at or near the crossing plane, and configured to trim theplurality of secondary electron beams.

In some embodiment, a method performed by a secondary imaging system toform images of a sample is provided. The method includes deflectingsecondary electron beams by a plurality of crossover-forming deflectorsbeing positioned at or at least near one or more image planes of thesecondary imaging system to form a crossover area on a crossing plane.The method can also include trimming secondary electron beams by abeam-limit aperture plate with a beam-limit aperture positioned at or atleast near the crossing plane.

In some embodiments, a method to form images of a sample using anelectro-optical system is provided. The method includes generating amagnetic field to immerse a surface of the sample and projecting aplurality of primary electron beams onto the surface of the sample by aprimary projection imaging system, wherein the plurality of primaryelectron beams pass through the magnetic field and generate a pluralityof secondary electron beams from the sample. The method also includesprojecting, by a secondary imaging system, the plurality of secondaryelectron beams onto an electron detection device to obtain the images,wherein at least some of the plurality of secondary electron beams aredeflected for creating a crossover area and are trimmed at or at leastnear to the crossover area.

In some embodiments, a non-transitory computer readable medium isprovided. The non-transitory computer readable medium stores a set ofinstructions that is executable by one or more processors of acontroller causing the controller to perform a method to form images ofa sample using an electro-optical system. The method includes providinginstructions to cause a plurality of crossover-forming deflectors beingpositioned at or at least near one or more image planes of the system todeflect secondary electron beams to form a crossover area. The methodalso includes providing instructions to cause a beam-limit aperture totrim the deflected secondary electron beams at or at least near to thecrossover area.

In some embodiments, a non-transitory computer readable medium isprovided. The non-transitory computer readable medium stores a set ofinstructions that is executable by one or more processors of acontroller causing the controller to perform a method forming images ofa sample. The method includes instructing an objective lens to generatea magnetic field to immerse a surface of the sample and instructing aprimary imaging system to project a plurality of primary electron beamsonto the surface of the sample, wherein the plurality of primaryelectron beams pass through the magnetic field and generate a pluralityof secondary electron beams from the sample. The method also includesinstructing a secondary imaging system to project the plurality ofsecondary electron beams onto an electron detection device to obtain theimages, wherein at least some of the plurality of secondary electronbeams are deflected for creating a crossover area and are trimmed at orat least near the crossover area.

Other advantages of the present invention will become apparent from thefollowing description taken in conjunction with the accompanyingdrawings wherein are set forth, by way of illustration and example,certain embodiments of the present invention.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 is a schematic diagram illustrating an exemplary electron beaminspection (EBI) system, consistent with embodiments of the presentdisclosure.

FIG. 2 is a schematic diagram illustrating an exemplary electron beamtool that is part of the exemplary electron beam inspection system ofFIG. 1, consistent with embodiments of the present disclosure.

FIG. 3A is a schematic diagram illustrating an exemplary configurationof objective lens having a magnetic lens and an electrostatic lens.

FIG. 3B illustrates exemplary magnetic field and electrostatic field ofthe magnetic lens and the electrostatic lens, respectively, of FIG. 3A.

FIG. 3C is a schematic diagram illustrating an exemplary secondaryelectron beams that have been influenced by the objective lens.

FIG. 3D is a cross sectional diagram of a beam-limit aperture plate of asecondary imaging system.

FIG. 4A is a schematic diagram illustrating an exemplary deflector of asecondary imaging system, consistent with embodiments of the presentdisclosure.

FIG. 4B is a schematic diagram illustrating an exemplary crossover ofsecondary electron beams at a secondary imaging system, consistent withembodiments of the present disclosure.

FIG. 4C is a cross sectional diagram of a beam-limit aperture plate of asecondary imaging system, consistent with embodiments of the presentdisclosure.

FIGS. 5A, 5B, and 5C are schematic diagrams illustrating exemplaryconfigurations of secondary imaging system with deflectors, consistentwith embodiments of the present disclosure.

FIG. 6 is a schematic diagram illustrating an exemplary configuration ofsecondary imaging system with deflectors configured in multipledeflector arrays, consistent with embodiments of the present disclosure.

FIG. 7 is a flow chart illustrating an exemplary method for formingimages of a surface of a sample, consistent with embodiments of thepresent disclosure.

FIG. 8 is a flow chart illustrating an exemplary method for formingimages of a sample using an electro-optical system, consistent withembodiments of the present disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments, examplesof which are illustrated in the accompanying drawings. The followingdescription refers to the accompanying drawings in which the samenumbers in different drawings represent the same or similar elementsunless otherwise represented. The implementations set forth in thefollowing description of exemplary embodiments do not represent allimplementations consistent with the invention. Instead, they are merelyexamples of apparatuses and methods consistent with aspects related tothe invention as recited in the appended claims.

Relative dimensions of components in drawings may be exaggerated forclarity. Within the following description of drawings the same or likereference numbers refer to the same or like components or entities, andonly the differences with respect to the individual embodiments aredescribed.

Without limiting the scope of the protection, all the description anddrawings of the embodiments will exemplarily be referred to as anelectron beam. However, the embodiments are not used to limit thepresent invention to specific charged particles.

Reference is now made to FIG. 1, which is a schematic diagramillustrating an exemplary electron beam inspection (EBI) system,consistent with embodiments of the present disclosure. As shown in FIG.1, charged particle beam inspection system 1 includes a main chamber 10,a load/lock chamber 20, an electron beam tool 100, and an equipmentfront end module (EFEM) 30. Electron beam tool 100 is located withinmain chamber 10.

EFEM 30 includes a first loading port 30 a and a second loading port 30b. EFEM 30 may include additional loading port(s). First loading port 30a and second loading port 30 b may, for example, receive wafer frontopening unified pods (FOUPs) that contain wafers (e.g., semiconductorwafers or wafers made of other material(s)) or samples to be inspected(wafers and samples are collectively referred to as “wafers” hereafter).One or more robot arms (not shown) in EFEM 30 transport the wafers toload/lock chamber 20.

Load/lock chamber 20 is connected to a load/lock vacuum pump system (notshown), which removes gas molecules in load/lock chamber 20 to reach afirst pressure below the atmospheric pressure. After reaching the firstpressure, one or more robot arms (not shown) transport the wafer fromload/lock chamber 20 to main chamber 10. Main chamber 10 is connected toa main chamber vacuum pump system (not shown), which removes gasmolecules in main chamber 10 to reach a second pressure below the firstpressure. After reaching the second pressure, the wafer is subject toinspection by electron beam tool 100. While the present disclosureprovides examples of main chamber 10 housing an electron beam inspectionsystem, it should be noted that aspects of the disclosure in theirbroadest sense, are not limited to a chamber housing an electron beaminspection system. Rather, it is appreciated that the foregoingprinciples may also be applied to other tools that operate under thesecond pressure.

Reference is now made to FIG. 2, which is a schematic diagramillustrating an exemplary electron beam tool 100 that is part of theexemplary electron beam inspection system of FIG. 1, consistent withembodiments of the present disclosure. An electron beam tool 100 (alsoreferred to herein as apparatus 100) comprises an electron source 101, agun aperture plate 171, a condenser lens 110, a source conversion unit120, a primary projection optical system 130, a sample 8 with samplesurface 7, a secondary imaging system 150, and an electron detectiondevice 140M. Primary projection optical system 130 may comprise anobjective lens 131. Electron detection device 140M may comprise aplurality of detection elements 140_1, 140_2, and 140_3. A beamseparator 160 and a deflection scanning unit 132 may be placed insideprimary projection optical system 130.

Electron source 101, gun aperture plate 171, condenser lens 110, sourceconversion unit 120, beam separator 160, deflection scanning unit 132,and primary projection optical system 130 may be aligned with a primaryoptical axis 100_1 of apparatus 100. Secondary imaging system 150 andelectron detection device 140M may be aligned with a secondary opticalaxis 150_1 of apparatus 100.

Electron source 101 may comprise a cathode (not shown) and an extractorand/or anode (not shown), in which, during operation, electron source101 is configured to emit primary electrons from the cathode and theprimary electrons are extracted and/or accelerated by the extractorand/or the anode to form a primary electron beam 102 that forms aprimary beam crossover (virtual or real) 101 s. Primary electron beam102 may be visualized as being emitted from primary beam crossover 101s.

Source conversion unit 120 may comprise an image-forming element array(not shown in FIG. 2) and a beam-limit aperture array (not shown in FIG.2). An example of source conversion unit 120 may be found in U.S. Pat.No. 9,691,586; U.S. application Ser. No. 15/216,258; and InternationalApplication No. PCT/EP2017/084429, all of which are incorporated byreference in their entireties. The image-forming element array maycomprise a plurality of micro-deflectors and/or micro-lenses toinfluence a plurality of primary beamlets 102_1, 102_2, 102_3 of primaryelectron beam 102 and to form a plurality of parallel images (virtual orreal) of primary beam crossover 101 s, one for each of the primarybeamlets 102_1, 201_2, 102_3. The beam-limit aperture array may beconfigured to limit diameters of individual primary beamlets 102_1,102_2, and 102_3. FIG. 2 shows three primary beamlets 102_1, 102_2, and102_3 as an example, and it is appreciated that source conversion unit120 may be configured to form any number of primary beamlets.

Condenser lens 110 is configured to focus primary electron beam 102.Condenser lens 110 may further be configured to adjust electric currentsof primary beamlets 102_1, 102_2, and 102_3 downstream of sourceconversion unit 120 by varying the focusing power of condenser lens 110.Alternatively, the electric currents may be changed by altering theradial sizes of beam-limit apertures within the beam-limit aperturearray corresponding to the individual primary beamlets. Objective lens131 (further explained below) may be configured to focus beamlets 102_1,102_2, and 102_3 onto a sample 8 for inspection and may form, in thecurrent embodiments, three probe spots 102_1 s, 102_2 s, and 102_3 s onsurface 7. Gun aperture plate 171, in operation, is configured to blockoff peripheral electrons of primary electron beam 102 to reduce Coulombeffect. The Coulomb effect may enlarge the size of each of probe spots102_1 s, 102_2 s, and 102_3 s of primary beamlets 102_1, 102_2, 102_3,and therefore deteriorate inspection resolution.

Beam separator 160 may, for example, be a Wien filter comprising anelectrostatic deflector generating an electrostatic dipole field E1 anda magnetic dipole field B1 (both of which are not shown in FIG. 2). Inoperation, beam separator 160 may be configured to exert anelectrostatic force by electrostatic dipole field E1 on individualelectrons of primary beamlets 102_1, 102_2, and 102_3. The electrostaticforce is equal in magnitude but opposite in direction to the magneticforce exerted by magnetic dipole field B1 of beam separator 160 on theindividual electrons. Primary beamlets 102_1, 102_2, and 102_3 maytherefore pass at least substantially straight through beam separator160 with at least substantially zero deflection angles.

Deflection scanning unit 132, in operation, is configured to deflectprimary beamlets 102_1, 102_2, and 102_3 to scan probe spots 102_1 s,102_2 s, and 102_3 s across individual scanning areas in a section ofsurface 7. In response to incidence of primary beamlets 102_1, 102_2,and 102_3 at probe spots 102_1 s, 102_2 s, and 102_3 s, electrons emergefrom sample 8 and generate three secondary electron beams 102_1 se,102_2 se, and 102_3 se, which, in operation, are emitted from sample 8.Each of secondary electron beams 102_1 se, 102_2 se, and 102_3 setypically comprise electrons having different energies includingsecondary electrons (having electron energy ≤50 eV) and backscatteredelectrons (having electron energy between 50 eV and the landing energyof primary beamlets 102_1, 102_2, and 102_3). Beam separator 160 isconfigured to deflect secondary electron beams 102_1 se, 102_2 se, and102_3 se towards secondary imaging system 150. Secondary imaging system150 subsequently focuses secondary electron beams 102_1 se, 102_2 se,and 102_3 se onto detection elements 140_1, 140_2, and 140_3 of electrondetection device 140M. Detection elements 140_1, 140_2, and 140_3 arearranged to detect corresponding secondary electron beams 102_1 se,102_2 se, and 102_3 se and generate corresponding signals which are sentto signal processing units (not shown), e.g. to construct images of thecorresponding scanned areas of sample 8.

Reference is now made to FIG. 3A, which illustrates an exemplaryconfiguration of objective lens 131 of FIG. 2. To obtain a higherresolution of images formed by primary beamlets 102_1, 102_2, and 102_3,objective lens 131 may be an electromagnetic compound lens in which thesample may be immersed in the magnetic field of objective lens 131.

In some embodiments, objective lens 131 includes a magnetic lens 131Mand an electrostatic lens 131E. Magnetic lens 131M includes amagnetic-circuit gap G1 between pole-pieces 131_mp1 and 131_mp2.Magnetic lens 131M is configured to focus each primary beamlet 102_1,102_2, and 102_3 at relatively low aberrations to generate relativelysmall probe spots 102_1 s, 102_2 s, 102_3 s of each of primary beamlets102_1, 102_2, 102_3 on sample 8.

In some embodiments, electrostatic lens 131E is formed by pole-piece131_mp1 and/or pole-piece 131_mp2 of magnetic lens 131M, field-controlelectrode 131_e1, and sample 8. Electrostatic lens 131E is configured toinfluence the landing energy of each primary beamlet 102_1, 102_2, and102_3 to ensure that the primary electrons land on sample 8 at arelatively low kinetic energy and pass through the apparatus with arelatively high kinetic energy.

In some embodiments, objective lens 131 is configured to be an“immersion lens.” As a result, sample 8 is immersed both in anelectrostatic field E (electrostatic immersion) of electrostatic lens131E and a magnetic field B (magnetic immersion) of magnetic lens 131M,as shown in FIG. 3B. Magnetic immersion may occur when a ratio of themagnetic field strength on surface 7 and the peak magnetic fieldstrength is larger than a preset ratio value, such as 5%. In someembodiments, sample 8 may not be immersed in magnetic field B.

Electrostatic immersion and magnetic immersion may reduce aberrations ofobjective lens 131. As electrostatic and magnetic fields get stronger,the aberrations of objective lens 131 become smaller. Electrostaticfield E, however, should be limited to within a safe range in order toavoid discharging or arcing on sample 8. Field-control electrode 131_e1of FIG. 3A may be configured to control electrostatic field E to staywithin that safe range. Due to this limitation of the field strength ofelectrostatic field E, further enhancement of the magnetic fieldstrength in the current immersion configuration allows to further reducethe aberrations of objective lens 131, and thereby improve imageresolution.

In some embodiments, primary beamlets 102_1, 102_2, 102_3 may arrive onsample surface 7 in an at least substantially perpendicular direction.Magnetic immersion, however, may influence the landing angles of allprimary beamlets landing on sample surface 7. In particular, magneticfield B may cause each electron in a primary beamlet to obtain anangular velocity θ⁽¹⁾, as shown in equation (1) below:

$\begin{matrix}{{r^{2}\theta^{(1)}} = {{\frac{1}{2}\frac{e}{m}r^{2}B} + C}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

wherein C is a constant related to an initial angular velocity of theelectron, r is a position shift from optical axis of objective lens 131,and e and m are the charge and the mass of the electron, respectively.For the electron to land on surface 7 in a perpendicular manner, angularvelocity θ⁽¹⁾ must be zero on sample surface 7.

In some embodiments, magnetic lens 131M is configured to operate in anon-magnetic immersion mode, and magnetic field B is zero (orsubstantially zero) or below the preset ratio value on surface 7. If anelectron enters magnetic field B along a meridional path, itscorresponding constant C is zero and its angular velocity θ⁽¹⁾ will bezero or substantially zero on sample 8. Furthermore, if an electronenters magnetic field B along a specific meridional path, it can crossoptical axis 100_1 of magnetic lens 131M before entering magnetic lens131M and perpendicularly land on sample 8. In other words, objectivelens 131 has a real front focal point on its front focal plane. When thechief rays (or center rays) of off-axis primary beamlets 102_2 and 102_3enter objective lens 131 along some specific meridional paths, the chiefrays can pass through the real front focal point and off-axis primarybeamlets 102_2 and 102_3 can land on surface 7 perpendicular.Accordingly, primary beamlets 102_1, 102_2 and 102_3 overlap together onthe front focal plane and form a relatively sharp beamlet crossovercentering at the real front focal point.

In other embodiments, magnetic lens 131M is configured to operate inmagnetic immersion mode in which magnetic field B is not zero on surface7. Therefore, angular velocity θ⁽¹⁾ of an electron may be zero (orsubstantially zero) on sample 8 if its corresponding constant C is notzero when the electron enters magnetic field B and complies with thecondition in equation (2):

$\begin{matrix}{C = {{- \frac{1}{2}}\frac{e}{m}r^{2}B}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

When C is not equal to zero, the electron enters magnetic field B alonga skew path and cannot cross optical axis 100_1 of magnetic lens 131Mbefore entering magnetic field B. Hence, an electron can perpendicularlyland on sample 8 only if entering magnetic field B along a specific skewpath, and the electron cannot really cross optical axis 100_1 duringpassing through magnetic field B. Accordingly objective lens 131 has avirtual front focal point 1. When the chief rays (or center rays) ofoff-axis primary beamlets 102_2 and 102_3 enter objective lens 131 alongsome specific skew paths, they can virtually pass through virtual frontfocal point 1 and land on sample surface 7 perpendicular. Under thisscenario, off-axis primary beamlets 102_2 and 102_3 are closest to eachother on principal plane 2 of objective lens 131, and each off-axisprimary beamlets 102_2 and 102_3 has a radial shift 3 from optical axis100_1. The primary beamlets 102_1-102_3 therefore only partially overlapwith each other on principal plane 2 and form a partial-overlap beamletcrossover on principal plane 2. Moreover, radial shift 3 increases asmagnetic field B on surface 7 increases. Current density is lower in thepartial-overlap beamlet crossover than in the foregoing sharp beamletcrossover. Therefore, the Coulomb interaction between primary beamlets102_1, 102_2, and 102_3 in magnetic immersion mode is relatively low,thereby further contributing to the small sizes of probe spots102_1S-102_3S.

As mentioned above, when objective lens 131 operates in magneticimmersion mode, chief rays of off-axis primary beamlets 102_2 and 103_3are required to enter magnetic field B or objective lens 131 along somespecific skew paths so that off-axis primary beamlets 102_2 and 102_3can land on sample 8 perpendicularly. The corresponding constants C ofthe off-axis chief rays are determined by equation (2). A deflector canbe used to adjust a primary beamlet to enter objective lens 131 alongits corresponding specific skew path. For example, micro-deflectors insource-conversion unit 120 may be used to individually deflect off-axisprimary beamlets 102_2 and 102_3 to eliminate the angular velocities ofthose beamlets on surface 7 so that those beamlets land on surface 7perpendicularly.

For secondary electron emissions from a sample surface, the angulardistribution follows Lambert's law. That is, the angular distribution isproportional to cos ϕ, where ϕ is the emission angle relative to thesurface normal. Therefore, the chief rays of off-axis secondary electronbeams 102_2 se and 102_3 se in FIG. 3A perpendicularly depart fromsample surface 7 and are thereby meridional (i.e., C=0). Accordingly,the chief rays of off-axis secondary electron beams 102_2 se, 102_3 sehave angular velocities as shown in equation (3).

$\begin{matrix}{\theta^{(1)} = {\frac{1}{2}\frac{e}{m}B}} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$

In some embodiments in which magnetic field B is zero at the samplesurface, objective lens 131 works in non-magnetic immersion mode.Accordingly, angular velocity θ⁽¹⁾ of the emerging secondary electronswould also be zero on the sample surface. In such embodiments, the chiefrays of off-axis secondary electron beams 102_2 se and 102_3 se arestill meridional after exiting objective lens 131, and may be able tocross optical axis 150_1 of secondary imaging system 150 (not shown inFIG. 2). Furthermore, the chief rays can cross optical axis 150_1 at asame place (if aberrations are not considered) in secondary imagingsystem 150. As such, secondary electron beams 102_1 se-102_3 se may beconfigured to overlap at a common area of crossing and therefore form arelatively sharp secondary beam crossover. The plane where the commonarea of crossing or secondary beam crossover is located is referred toas a crossing plane.

When objective lens operates in the non-magnetic immersion mode, asecondary beam-limit aperture plate (e.g., secondary beam-limit apertureplate 155 shown in FIGS. 3D, 4C) with a secondary beam-limit aperture(e.g., secondary beam-limit aperture 155A shown in FIGS. 3D, 4C) insecondary imaging system 150 may be placed at the crossing plane suchthat secondary electron beams 102_1 se-102_3 se may pass through thesecondary beam-limit aperture in a substantially same manner similar tothe exemplary embodiment of FIG. 4C. In such a scenario, the secondarybeam-limit aperture cuts off peripheral electrons of secondary electronbeams 102_1 se-102_3 se in a substantially uniform manner. By cuttingoff the peripheral electrons in the substantially uniform manner, thesecondary beam-limit aperture may allow secondary electron beams 102_1se-102_3 se to have the same or at least similar passing rates andtherefore assist electron detection device 140M with relatively highcollection efficiency uniformity across the detected secondary electronbeams 102_1 se-102_3 se. Moreover, cutting off the peripheral electionsin the substantially uniform manner (e.g., using the same blockingeffects in any radial directions) reduces cross talk of the secondaryelectron beams on electron detection device 140M.

In alternative embodiments when magnetic field B is not zero atsubstrate surface 7, objective lens 131 operates in magnetic immersionmode. Accordingly, angular velocity θ⁽¹⁾ of the emerging secondaryelectrons is not zero on the sample surface. The chief rays of off-axissecondary electron beams 102_2 se and 102_3 se have angular velocitiesafter exiting objective lens, and will be skew rays.

Reference is now made to FIGS. 3C and 3D, which illustrate how operatingunder magnetic immersion mode may affect secondary electron beams. Inparticular, FIG. 3C shows a schematic diagram of exemplary secondaryelectron beams that emerge from surface 7 of sample 8 and are influencedby objective lens 131 (not shown), while FIG. 3D shows a cross sectionaldiagram of a beam-limit aperture plate of a secondary imaging system151. In these alternative embodiments, the chief rays of off-axissecondary beams 102_2 se and 102_3 se become skewed such that they nolonger cross optical axis 150_1 of secondary imaging system 150. Forexample, as shown in FIG. 3C, secondary electron beams 102_1 se-102_3 seonly partially overlap in one or more crossing planes (such as crossingplane 4) in secondary imaging system 150 and form a partial-overlapsecondary beam crossover at each of the crossing planes. At crossingplane 4, the chief rays of off-axis secondary electron beams 102_2 seand 102_3 se have a radial shift 5 from optical axis 150_1. As magneticfield B increases, the separation of off-axis secondary electron beams102_2 se and 102_3 se from optical axis 150_1 (and on-axis secondaryelectron beam 102_1 se) becomes larger, thereby further reducing theamount of overlap across secondary electron beams 102_1 se-102_3 se.

In these embodiments, as shown in FIG. 3D, secondary beam-limit aperture155A in secondary imaging system 150 is placed at onepartially-overlapping secondary beam crossover on the correspondingcrossing plane. As shown in FIG. 3D, the lack of a common crossing pointof the chief rays of the secondary electron beams causes secondbeam-limit aperture 155A to cut off different parts of the peripheralelectrons from different secondary electron beams 102_1 se-102_3 se.On-axis secondary electron beam 102_1 se has a higher passing ratecompared to off-axis secondary electron beams 102_2 se and 102_3 se.While peripheral electrons of on-axis secondary electron beam 102_1 seare uniformly cut off in all azimuths, the peripheral electrons ofoff-axis secondary electron beams 102_2 se and 102_3 se cannot beuniformly cut off in all azimuths. For example, for secondary electronbeam 102_2 se, more peripheral electrons are cut off on the right sidethan on the left side. Thus, when using an objective lens in animmersion configuration, differences of collection efficiencies ofsecondary electron beams 102_1 se-102_3 se may occur and the cross-talkamong secondary electron beams 102_1 se-102_3 se may be larger (whencompared to objective lens operating in the non-magnetic immersionmode).

Accordingly, when objective lens 131 operates in magnetic immersionmode, the secondary imaging system has a reduced Coulomb Effect,although collection efficiency uniformity and cross-talk acrosssecondary electron beams at electron detection device 140M may benegatively impacted. The disclosed embodiments provide a system, whileoperating in magnetic immersion mode, that is configured to minimizeradial shifts of off-axis secondary electron beams at a common area ofcrossing such that a secondary beam-limit aperture may be placed at thecommon area of crossing to evenly reduce the peripheral electrons forall secondary electron beams. The disclosed embodiments are configuredto reduce the cross-talk and improve collection efficiency uniformityfor the secondary electron beams.

Reference is now made to FIG. 4A, which illustrates an exemplarydeflector used by a secondary imaging system, consistent withembodiments of the present disclosure. The secondary imaging system(e.g., secondary imaging system 150 of FIG. 2) may include a deflectorD1 to deflect a secondary electron beam 102_2 se to reduce the angularvelocity of the chief ray of secondary electron beam 102_2 se. As aresult, the trajectory of the secondary electron beam may be changedfrom a skewed trajectory to a meridional trajectory. As a result, thedeflected secondary electron beam may be configured to cross secondaryoptical axis 150_1 at a designed location.

For example, deflector D1 may be placed at least near an image plane P1of a lens or lens group of secondary imaging system 150 and may beoptically aligned with secondary electron beam 102_2 se. Such deflectorD1 may, for example, be a multi-pole structure. For example, whendeflector D1 is a 4-pole structure, a zero voltage is applied to onepair of opposite poles, while two voltages of the same absolute valuebut opposite polarities are applied to the other pair of opposite poles.As the voltage increases on the opposite poles, the angle of deflectionof the beamlet increases as well.

As stated above, deflector D1 is placed at least near image plane P1. Insome embodiments, deflector D1 is placed at the image plane P1 of one ormore lenses of secondary imaging system 150. In the embodiment in whichdeflector D1 is near image plane P1, deflection of the secondaryelectron beam may influence a magnification of the secondary imagingsystem 150, but this influencing may be acceptable if the influencingremains within certain limits.

Deflector D1 may, for example, be part of a deflector array.Accordingly, for each secondary electron beam, a deflector may be placedat least near to an image plane and optically aligned with thecorresponding secondary electron beam. It is appreciated that for anon-axis secondary beam, a corresponding deflector may not be necessary.

The individual deflectors of the deflector array may be configured todeflect their corresponding secondary electron beams to cross secondaryoptical axis 150_1 at a desired crossing plane (not shown in FIG. 4A)and form a relatively sharp secondary beam crossover thereon. Thedistance between the deflector and the crossing plane may depend on thesecondary beam's deflection angle caused by the deflector. For example,as shown in FIG. 4A, deflector D1 deflects off-axis secondary electronbeam 102_2 se such that the chief ray of secondary electron beam 102_2se crosses secondary optical axis 150_1 at a desired crossing plane.

Reference is now made to FIGS. 4B-4C, which show embodiments that areconsistent with embodiments of the present disclosure. FIG. 4B is aschematic diagram illustrating an exemplary relatively sharp secondarybeam crossover of secondary electron beams on crossing plane 4 in asecondary imaging system, while FIG. 4C illustrates a cross sectionaldiagram of an exemplary beam-limit aperture plate on crossing plane 4.

In FIG. 4B, off-axis secondary electron beams 102_2 se and 102_3 se aredeflected by their corresponding deflectors (not shown) to crosssecondary optical axis 150_1 at crossing plane 4 and form a relativelysharp secondary beam crossover thereon with on-axis secondary electronbeam 102_1 se. As a result, any beam-limiting aperture placed at thesecondary beam crossover may be used to evenly cut off peripheralelectrons from the individual secondary electron beams to reducecross-talk and ensure similar collection efficiencies. This is shown inFIG. 4C.

FIG. 4C shows the cross sectional perspective on crossing plane 4 whensecondary electron beams pass through and cross over an opening ofsecondary beam-limit aperture 155A in beam-limit aperture plate 155 thatis placed on crossing plane 4 in FIG. 4B. In FIG. 4C, the dashed circleillustrates the overlapping secondary electron beams 102_1 se, 102_2 se,102_3 se on a surface of secondary beam-limit aperture plate 155. Asshown, secondary beam-limit aperture 155A cuts off peripheral electronsof secondary electron beams 102_1 se-102_3 se. While FIGS. 4B-4Cillustrate examples of a relatively sharp secondary beam crossoverformed by fully overlapping secondary beams 102_1 se-102_3 se on onecrossing plane, it is appreciated that one or more of the secondaryelectron beams may be offset from others on the crossing plane and thesecondary beam crossover may not be that sharp. The offset may keep atleast 80% of the electrons from the beam overlap with others.

In general, when the size of secondary beam-limit aperture 155Aincreases, the collection efficiencies of secondary electron beams 102_1se-102_3 se increase, and the collection efficiency differences andcross-talk among secondary electron beams 102_1 se-102_3 se increase aswell. When the collection efficiencies of secondary electron beams 102_1se-102_3 se increase, the inspection throughput of the multi-beaminspection (MBI) apparatus increases. On the other side, when thecollection efficiency differences of secondary electron beams 102_1se-102_3 se increase, the grey levels of images formed by secondaryelectron beams 102_1 se-102_3 se differ more, which require one or moreadditional processes to eliminate the inspection errors due to thedifference, thereby decreasing inspection throughput and deterioratingresolution of the MBI apparatus. When the cross-talk among secondaryelectron beams 102_1 se-102_3 se increases, the resolution of imagesformed by secondary electron beams 102_1 se-102_3 se reduces. That is,large cross-talk deteriorates inspection resolution of the MBIapparatus.

Reference is now made to FIGS. 5A, 5B, and 5C, which are schematicdiagrams illustrating exemplary configurations of the secondary imagingsystem comprising deflectors, consistent with embodiments of the presentdisclosure.

Secondary imaging system 150 may comprise a first set of one or morelenses 151 and a second set of one or more lenses 152 (shown in FIGS.5A, 5B, and 5C) that are arranged between beam separator 160 andelectron detection device 140M, in which electron detection device 140Mis positioned closer to second set of one or more lenses 152 than firstset of one or more lenses 151. Each set comprises one or more lensesthat together perform an optical function, such as magnifying, zooming,and image anti-rotating etc. For example, first set 151 may beconfigured to have a zooming function to reduce magnification variationsof multiple secondary electron beams on one image plane. In particular,each set of one or more lenses 151 and 152 may be configured to form animage plane for secondary electron beams. For example, first set of oneor more lenses 151 may be arranged to form an image plane P1 forsecondary electron beams. As a result, deflectors may beneficially bepositioned at or near image plane P1 to deflect secondary electronbeams. Similarly, second set of one or more lenses 152 may be arrangedto form an image plane P2 for secondary electron beams, such that, e.g.electron detection device 140M may be positioned at or near image planeP2 to form multiple images of sample 8, one image for each of thesecondary electron beams.

In some embodiments, a crossover-forming deflector array 190 withmultiple crossover-forming deflectors is arranged at or near image planeP1 after first set 151 along secondary optical axis 150_1. Eachcrossover-forming deflector in crossover-forming deflector array 190 maybe aligned with a corresponding one of secondary electron beams 102_1se, 102_2 se, 102_3 se and may be configured to deflect thecorresponding secondary electron beam to cross secondary optical axis150_1 on crossing plane 4. Each crossover-forming deflector may, forexample, be deflector D1 of FIG. 4A.

While FIGS. 5A-5C show crossover-forming deflectors in crossover-formingdeflector array 190 associated with off-axis secondary electron beams102_2 se and 102_3 se, it is appreciated that an additional deflectormay also be used by on-axis secondary electron beam 102_1 se.

The deflection of secondary electron beams 102_1 se-102_3 se ensuresecondary electron beams 102_1 se-102_3 se form a relatively sharpsecondary beam crossover at or near crossing plane 4. Beam-limitaperture plate 155 with a beam-limit aperture 155A is located at or nearone crossing plane (e.g. crossing plane 4). While peripheral electronsof secondary electron beams 102_1 se-102_3 se are blocked off bybeam-limit aperture plate 155, the center electrons of secondaryelectron beams 102_1 se-102_3 se pass through beam-limit aperture 155A.

The placement of beam-limit aperture plate 155 and crossing plane 4 maybe designed at various locations, e.g. depending on the deflectionangles and/or location of crossover-forming deflector array 190. Forexample, as shown in FIG. 5A, the placement of beam-limit aperture plate155 and crossing plane 4 is located between first set of one or morelenses 151 and second set of one more lenses 152. In some embodiments,such as the exemplary configuration shown in FIG. 5B, the placement ofbeam-limit aperture plate 155 and crossing plane 4 is located insidesecond set of one more lenses 152. In some other embodiments, such asthe exemplary configuration shown in FIG. 5C, the placement ofbeam-limit aperture plate 155 and crossing plane 4 is located betweensecond set of one more lenses 152 and electron detection device 140M. Abenefit of the configuration as shown in FIG. 5C is that the deflectionangles of the individual crossover-forming deflectors in thecrossover-forming deflector array 190 are relatively small. A benefit ofsuch small deflection angles may be that the crossover-forming deflectorarray 190 may be easier to manufacture because lower electric excitationto each crossover-forming deflector is required. Furthermore, suchconfiguration may be more power efficient.

Secondary imaging system 150 is configured to focus secondary electronbeams 102_1 se, 102_2 se, and 102_3 se from scanned regions of samplesurface 7 onto corresponding detection elements 140_1, 140_2, and 140_3of electron detection device 140M at image plane P2, and form secondarybeam spots 102_1 seP, 102_2 seP and 102_3 seP thereon respectively.Detection elements 140_1, 140_2, and 140_3 are configured to detectcorresponding secondary electron beams 102_1 se, 102_2 se, and 102_3 seand generate corresponding signals used to construct images of thecorresponding scanned areas of sample surface 7. Based on the disclosedembodiments provided in secondary imaging system 150, the use ofcrossover-forming deflector array 190 in secondary imaging system 150enables a reduction of the cross-talk between adjacent secondaryelectron beams, while improving the collection efficiency uniformity forsecondary electron beams 102_1 se-102_3 se.

Reference is now made to FIG. 6, which is a schematic diagramillustrating an exemplary alternative configuration of secondary imagingsystem 150 comprising multiple crossover-forming deflector arrays todeflect secondary electron beams 102_1 se-102_3 se, consistent withembodiments of the present disclosure.

Secondary imaging system 150 includes multiple sets of one more lenses151, 152, 153 arranged between beam separator 160 and electron detectiondevice 140M. In these embodiments, first set of one more lenses 151 isconfigured to focus secondary electron beams 102_1 se-102_3 se ontoimage plane P1 arranged between first set of one more lenses 151 andsecond set of one more lenses 152. Second set of one more lenses 152subsequently is configured to focus secondary electron beams 102_1se-102_3 se onto image plane P2 arranged between second set of one morelenses 152 and a third set of one more lenses 153. Third set of one morelenses 153 finally focuses secondary electron beams 102_1 se-102_3 seonto image plane P3 where electron detection device 140M is positioned.

In some embodiments, secondary imaging system 150 may comprise more thanone crossover-forming deflector array 190-1 and 190-2, with each arrayhaving one or more crossover-forming deflectors (e.g., deflector D1 ofFIG. 4A). Crossover-forming deflector arrays 190-1 and 190-2 are placedat image planes P1 and P2, respectively. Crossover-forming deflectorarray 190-1 comprises crossover-forming deflectors that are aligned withand deflect first group (102_2 se) of secondary electron beams 102_1se-102_3 se on image plane P1, while not influencing other groups ofsecondary electron beams to pass through. Crossover-forming deflectorarray 190-2 comprises crossover-forming deflectors that are aligned withand deflect second group (102_3 se) of secondary electron beams 102_1se-102_3 se on image plane P2, while not influencing other groups ofsecondary electron beams to pass through. The individual deflectionangles of the crossover-forming deflectors in crossover-formingdeflector array 190_1 and 190_2 are configured such that all secondaryelectron beams 102_1 se-102_3 se cross secondary optical axis 150_1 atcrossing plane 4 and form a relatively sharp crossover thereon. Asstated above, it is appreciated that one of crossover-forming deflectorarrays 190-1 and 190-2 may have a deflector corresponding to secondaryelectron beam 102_1 se.

The placement of beam-limit aperture plate 155 and crossing plane 4 maybe designed at various locations within secondary image system 150. Forexample, along secondary optical axis 150_1, the placement of beam-limitaperture plate 155 and crossing plane 4 may be placed between second andthird sets of one or more lenses 152 and 153, inside third set of one ormore lenses 153, or after third set of one or more lenses 153 and beforeelectron detection device 140M.

FIG. 7 is a flow chart illustrating an exemplary method 700 for formingimages of a surface of a sample, consistent with embodiments of thepresent disclosure. Method 700 may be performed by a secondary imagingsystem (e.g., secondary imaging system 150 of FIGS. 5A, 5B, 5C, and 6)after acquiring secondary electrons beams from sample.

In step 710, the secondary electron beams are deflected by a pluralityof crossover-forming deflectors being positioned at or at least near oneor more image planes to form a crossover area on a crossing plane. Thesecondary electron beams that are deflected may be all secondaryelectron beams acquired by the secondary imaging system or may beoff-axis secondary electron beams.

In some embodiments, the secondary electron beams may be deflected at orat least near one image plane. Prior to being deflected, the secondaryelectron beams may pass through a first set of one or more lenses (e.g.,first set of one or more lenses 151), which form a first image plane(e.g., image plane P1). The deflecting of the secondary electron beamsoccurs at or at least near the first image plane.

In some other embodiments, the secondary electron beams may be deflectedat or at least near multiple image planes. In such embodiments, a firstgroup of secondary electron beams are deflected by a firstcrossover-forming deflector array being positioned at or at least nearthe first image plane, while a second group of secondary electron beamsare deflected by a second crossover-forming deflector array beingpositioned at or at least near a second image plane. Prior to beingdeflected, the first group of secondary electron beams may pass throughthe first set of one or more lenses (e.g., first set of one or morelenses 151), which form the first image plane (e.g., image plane P1).The deflecting of the first group of secondary electron beams occurs ator at least near the first image plane.

Prior to being deflected, the second group of secondary electron beamsmay pass through a second set of one or more lenses (e.g., second set ofone or more lenses 152), which form the second image plane (e.g., imageplane P2). The deflecting of the second group of secondary electronbeams occurs at or at least near the second image plane.

In step 720, secondary electron beams are trimmed by a beam-limitaperture (e.g., beam-limit aperture 155A) positioned at or at least nearthe crossing plane associated with the crossover area. The trimming ofthe secondary electron beams may involve the trimming of the peripheralelectrons of the secondary electron beams and may allow center electronsof secondary electron beams to pass through the beam-limit aperture.

In embodiments where the secondary electron beams are deflected at or atleast near one image plane, the trimming of the secondary electron beamsmay occur at the crossing plane prior to a second set of one or morelenses forming an image plane (e.g., image plane P2 shown in FIG. 5A),within the second set of one or more lenses forming the image plane(e.g., image plane P2 shown in FIG. 5B), or after the second set of oneor more lenses forming the image plane (e.g., image plane P2 shown inFIG. 5C).

In embodiments where the secondary electron beams are deflected atmultiple image planes (such as image planes P1 and P2 shown in FIG. 6),the trimming of the secondary electron beams may occur at the crossingplane prior to a third set of one or more lenses (e.g., third set of oneor more lenses 153) forming a third image plane (e.g., image plane P3),within the third set of one or more lenses forming the third imageplane, or after the third set of one or more lenses forming the thirdimage plane.

Reference is now made to FIG. 8, which is a flow chart illustrating anexemplary method 800 of forming images of a sample using anelectro-optical system, consistent with embodiments of the presentdisclosure. Method 800 may be performed by electron beam tool (e.g.,electron beam tool 100 of FIG. 2).

In step 810, a magnetic field is generated to immerse a surface of asample (e.g., surface 7 of sample 8). The magnetic field may begenerated by a magnetic lens (e.g., magnetic lens 131M of FIG. 3A) of anobjective lens (e.g., objective lens 131).

In step 820, a plurality of electron beams are projected through themagnetic field of the objective lens onto the surface, wherein theplurality of electron beams illuminates the surface of the sample togenerate secondary electron beams from the surface. These multiplesecondary electron beams pass through the objective lens and a secondaryimaging system to form a plurality of secondary electron beam spots onan electron detection device.

The formation of the plurality of secondary electron beam spots includesstep 830 involving a deflecting of secondary electron beams bydeflectors to create a crossover area and step 840 involving a trimmingof secondary electron beams by a beam-limit aperture. Steps 830 and 840may be similar to steps 710 and 720 of FIG. 7.

In step 850, the plurality of secondary electron beam spots is detectedby a plurality of detection elements (e.g., detection elements140_1-140_3) of the electron detection device (e.g., electron detectiondevice 140M) and form a plurality images of the sample.

The embodiments may further be described using the following clauses:

1. A crossover-forming deflector array of an electro-optical system fordirecting a plurality of electron beams onto an electron detectiondevice, the crossover-forming deflector array comprising:

a plurality of crossover-forming deflectors positioned at or at leastnear an image plane of a set of one or more electro-optical lenses ofthe electro-optical system, wherein each crossover-forming deflector isaligned with a corresponding electron beam of the plurality of electronbeams.

2. The crossover-forming deflector array of clause 1, wherein eachcrossover-forming deflector is configured to deflect a correspondingelectron beam so that all electron beams overlap to form a crossoverarea on a crossing plane.3. The crossover-forming deflector array of clause 1, wherein eachcrossover-forming deflector has a multi-pole structure.4. An electro-optical system for projecting a plurality of secondaryelectron beams from a sample onto respective electron detection surfacesin a multi-beam apparatus, the electro-optical system comprising:

a plurality of crossover-forming deflectors configured to create acrossover area on a crossing plane for the plurality of secondaryelectron beams, wherein each crossover-forming deflector of theplurality of crossover-forming deflectors is associated with acorresponding secondary electron beam of the plurality of secondaryelectron beams

a beam-limit aperture plate with one or more apertures, positioned at ornear the crossing plane, and configured to trim the plurality ofsecondary electron beams.

5. The electro-optical system of clause 4, wherein the plurality ofcrossover-forming deflectors are configured to deflect off-axissecondary electron beams of the plurality of secondary electron beams tothe crossover area.6. The electro-optical system any one of clauses 4 and 5, wherein theone or more apertures include a first aperture centered with thecrossover area.7. The electro-optical system of any one of clauses 4 to 6, wherein theone or more apertures are configured to have different sizes.8. The electro-optical system of any one of clauses 6 and 7, wherein thebeam-limit aperture plate is moveable to align a second aperture of theone or more apertures with the crossover area.9. The electro-optical system of any one of clauses 4 to 8, furthercomprising:

an electron detection device including the detection surfaces for theplurality of secondary electron beams to form a plurality of images ofthe sample.

10. The electro-optical system of clause 9, further comprising:

a first set of one or more lenses and a second set of one or more lensesaligned with an optical axis of the electro-optical system, wherein theelectron detection device is positioned closer to the second set thanthe first set.

11. The electro-optical system of clause 10, wherein the plurality ofcrossover-forming deflectors are positioned at or at least near a firstimage plane of the first set between the first set and the second set.12. The electro-optical system of clause 11, wherein the first set ofone or more lenses is configured to align at least some of the pluralityof secondary electron beams with a corresponding crossover-formingdeflector of the plurality of crossover-forming deflectors.13. The electro-optical system of any one of clauses 11 and 12, whereinthe crossing plane is positioned between the first set and the secondset.14. The electro-optical system of any one of clauses 11 and 12, whereinthe crossing plane is positioned between the electron detection deviceand the second set.15. The electro-optical system of any one of clauses 11 and 12, whereinthe crossing plane is positioned within the second set16. The electro-optical system of clause 10, further comprising a thirdset of one or more lenses aligned with the optical axis of theelectro-optical system and positioned between the second set and theelectron detection device, wherein a first set of crossover-formingdeflectors of the plurality of crossover-forming deflectors ispositioned at or at least near a first image plane of the first set ofone or more lenses between the first set of one more lenses and thesecond set of one or more lenses and a second set of crossover-formingdeflectors of the plurality of crossover-forming deflectors ispositioned at or at least near a second image plane of the second set ofone or more lenses between the second set of one or more lenses and thethird set of one or more lenses.17. The electro-optical system of clause 16, wherein the first set ofcrossover-forming deflectors and the second set of crossover-formingdeflectors are configured to deflect corresponding secondary electronbeams to overlap to form the crossover area at the crossing plane.18. The electro-optical system of clause 17, wherein the crossing planeis positioned between the electron detection device and the third set ofone or more lenses.19. The electro-optical system of clause 17, wherein the crossing planeis positioned within the third set of one or more lenses.20. The electro-optical system of any one of clauses 10 to 19, whereinat least one of the first set of one or more lenses and the second setof one or more lenses is configured to compensate a rotation of theplurality of secondary electron beams before each secondary electionbeam of the plurality of secondary electron beams reaches acorresponding crossover-forming deflector.21. The electro-optical system of any one of clauses 4 to 21, whereinthe electro-optical system is configured to image secondary electronbeams onto the detection surfaces under a presence of an objective lensconfigured to immerse the sample with a magnetic field of the objectivelens.22. The electro-optical system of any one of clauses 16 to 21, whereinthe first set of one or more lenses and the second set of one or morelenses are configured to align at least some of the plurality ofsecondary electron beams with a corresponding crossover-formingdeflector of the plurality of crossover-forming deflectors.23. The electro-optical system of clause 22, wherein at least one of thefirst set of one or more lenses and the second set of one or more lensesis configured to compensate a displacement of the plurality of secondaryelectron beams before each secondary electron beam of the plurality ofsecondary electron beams reaches a corresponding crossover-formingdeflector.24. A method performed by a secondary imaging system to form images of asample, the method comprising:

deflecting secondary electron beams by a plurality of crossover-formingdeflectors being positioned at or at least near one or more image planesof the secondary imaging system to form a crossover area on a crossingplane; and

trimming secondary electron beams by a beam-limit aperture plate with abeam-limit aperture positioned at or at least near the crossing plane.

25. The method of clause 24, wherein trimming secondary electron beamsincludes blocking off peripheral portion of each of the secondaryelectron beams by the beam-limit aperture plate.26. The method of any one of clauses 24 and 25, wherein trimmingsecondary electron beams allows central portion of each of the secondaryelectron beams to pass through the beam-limit aperture.27. The method of any one of clauses 24 to 26, further comprising:

forming, by a first set of one or more lenses, a first image plane ofthe one or more image planes; and

forming, by a second set of one or more lenses, a second image plane ofthe one or more image planes.

28. The method of clause 27, wherein deflecting secondary electron beamsby one or more deflectors being positioned at least near one or moreimage planes to form a crossover area further comprises:

deflecting secondary electron beams by the plurality ofcrossover-forming deflectors being positioned at or at least near thefirst image plane.

29. The method of any one of clauses 27 and 28, wherein trimmingsecondary electron beams occurs before the second set forms the secondimage plane for the secondary electron beams.30. The method of any one of clauses 27 and 28, wherein trimmingsecondary electron beams occurs after the second set forms the secondimage plane for the secondary electron beams.31. The method of any one of clauses 27 and 28, wherein trimmingsecondary electron beams occurs during the second set forming the secondimage plane for the secondary electron beams.32. The method of clause 27, further comprising:

forming, by a third set of one or more lenses, a third image plane ofthe one or more image planes.

33. The method of clause 32, wherein deflecting secondary electron beamsby one or more crossover-forming deflectors being positioned at or atleast near one or more image planes to form a crossover area furthercomprises:

deflecting a first set of secondary electron beams of the secondaryelectron beams by a first set of crossover-forming deflectors beingpositioned at or at least near the first image plane; and

deflecting a second set of secondary electron beams of the secondaryelectron beams by a second set of crossover-forming deflectors beingpositioned at or at least near the second image plane.

34. The method of any one of clauses 32 and 33, wherein trimmingsecondary electron beams occurs before the third set forms the thirdimage plane for the secondary electron beams.35. The method of any one of clauses 32 and 33, wherein trimmingsecondary electron beams occurs after the third set forms the thirdimage plane for the secondary electron beams.36. The method of any one of clauses 32 and 33, wherein trimmingsecondary electron beams occurs during the third set forming the thirdimage plane for the secondary electron beams.37. A method to form images of a sample using an electro-optical systemcomprising:

generating a magnetic field to immerse a surface of the sample;

projecting a plurality of primary electron beams onto the surface of thesample by a primary projection imaging system, wherein the plurality ofprimary electron beams pass through the magnetic field and generate aplurality of secondary electron beams from the sample; and

projecting, by a secondary imaging system, the plurality of secondaryelectron beams onto an electron detection device to obtain the images,wherein at least some of the plurality of secondary electron beams aredeflected for creating a crossover area and are trimmed at or at leastnear to the crossover area.

38. A non-transitory computer readable medium including a set ofinstructions that is executable by one or more processors of acontroller causing the controller to perform a method to form images ofa sample using an electro-optical system, the method comprising:

providing instructions to cause a plurality of crossover-formingdeflectors being positioned at or at least near one or more image planesof the system to deflect secondary electron beams to form a crossoverarea; and

providing instructions to cause a beam-limit aperture to trim thedeflected secondary electron beams at or at least near to the crossoverarea.

39. A non-transitory computer readable medium including a set ofinstructions that is executable by one or more processors of acontroller causing the controller to perform a method forming images ofa sample, the method comprising:

instructing an objective lens to generate a magnetic field to immerse asurface of the sample;

instructing a primary imaging system to project a plurality of primaryelectron beams onto the surface of the sample, wherein the plurality ofprimary electron beams pass through the magnetic field and generate aplurality of secondary electron beams from the sample; and

instructing a secondary imaging system to project the plurality ofsecondary electron beams onto an electron detection device to obtain theimages, wherein at least some of the plurality of secondary electronbeams are deflected for creating a crossover area and are trimmed at orat least near the crossover area.

It is appreciated that a controller of the multi-beam apparatus coulduse software to control the functionality described above. For example,the controller may send instructions to the aforementioned lenses togenerate an appropriate field (e.g., magnetic or electrostatic field),respectively. The controller may also send instructions to adjustvoltages to control the aforementioned deflector arrays. The softwaremay be stored on a non-transitory computer readable medium. Common formsof non-transitory media include, for example, a floppy disk, a flexibledisk, hard disk, solid state drive, magnetic tape, or any other magneticdata storage medium, a CD-ROM, any other optical data storage medium,any physical medium with patterns of holes, a RAM, a PROM, and EPROM, aFLASH-EPROM or any other flash memory, NVRAM, a cache, a register, anyother memory chip or cartridge, and networked versions of the same.

Although the present invention has been explained in relation to itspreferred embodiments, it is to be understood that other modificationsand variation can be made without departing the spirit and scope of theinvention as hereafter claimed.

What is claimed is:
 1. A crossover-forming deflector array of anelectro-optical system for directing a plurality of electron beams ontoan electron detection device, the crossover-forming deflector arraycomprising: a plurality of crossover-forming deflectors positioned at orat least near an image plane of a set of one or more electro-opticallenses of the electro-optical system, wherein each crossover-formingdeflector is aligned with a corresponding electron beam of the pluralityof electron beams.
 2. The crossover-forming deflector array of claim 1,wherein each crossover-forming deflector is configured to deflect acorresponding electron beam so that all electron beams overlap to form acrossover area on a crossing plane.
 3. The crossover-forming deflectorarray of claim 1, wherein each crossover-forming deflector has amulti-pole structure.
 4. An electro-optical system for projecting aplurality of secondary electron beams from a sample onto respectiveelectron detection surfaces in a multi-beam apparatus, theelectro-optical system comprising: a plurality of crossover-formingdeflectors configured to create a crossover area on a crossing plane forthe plurality of secondary electron beams, wherein eachcrossover-forming deflector of the plurality of crossover-formingdeflectors is associated with a corresponding secondary electron beam ofthe plurality of secondary electron beams; and a beam-limit apertureplate with one or more apertures, positioned at or near the crossingplane, and configured to trim the plurality of secondary electron beams.5. The electro-optical system of claim 4, wherein the plurality ofcrossover-forming deflectors are configured to deflect off-axissecondary electron beams of the plurality of secondary electron beams tothe crossover area.
 6. The electro-optical system of claim 4, whereinthe one or more apertures include a first aperture centered with thecrossover area.
 7. The electro-optical system of claim 4, wherein theone or more apertures are configured to have different sizes.
 8. Theelectro-optical system of claim 6, wherein the beam-limit aperture plateis moveable to align a second aperture of the one or more apertures withthe crossover area.
 9. The electro-optical system of claim 4, furthercomprising: an electron detection device including the detectionsurfaces for the plurality of secondary electron beams to form aplurality of images of the sample.
 10. The electro-optical system ofclaim 9, further comprising: a first set of one or more lenses and asecond set of one or more lenses aligned with an optical axis of theelectro-optical system, wherein the electron detection device ispositioned closer to the second set than the first set.
 11. Theelectro-optical system of claim 10, wherein the plurality ofcrossover-forming deflectors are positioned at or at least near a firstimage plane of the first set between the first set and the second set.12. The electro-optical system of claim 11, wherein the first set of oneor more lenses is configured to align at least some of the plurality ofsecondary electron beams with a corresponding crossover-formingdeflector of the plurality of crossover-forming deflectors.
 13. Theelectro-optical system of claim 11, wherein the crossing plane ispositioned between the first set and the second set.
 14. Theelectro-optical system of claim 11, wherein the crossing plane ispositioned between the electron detection device and the second set. 15.A method performed by a secondary imaging system to form images of asample, the method comprising: deflecting secondary electron beams by aplurality of crossover-forming deflectors being positioned at or atleast near one or more image planes of the secondary imaging system toform a crossover area on a crossing plane; and trimming secondaryelectron beams by a beam-limit aperture plate with a beam-limit aperturepositioned at or at least near the crossing plane.